Project Summary Abnormal vascular homeostasis manifests itself in a variety of guises including neointima formation and reduced vascular integrity combined with increased permeability. Neointima growth underlies a number of common diseases including transplant vasculopathy, post- angioplasty and vascular graft restenosis, hypertension, and atherosclerosis among others, while vascular integrity is equally important in a number of systemic and local disorders associated with inflammation and tissue injury. Both of these key facets of vascular pathology tend to be viewed as distinctly different entities that have little in common with each other. Despite decades of investigations, the origin of neointima still remains controversial. Our preliminary data suggest that one potentially important contributor to neointima formation is the Endo-MT process. Endo-MT is thought to result in endothelial cells (EC) trans-differentiating into mesenchymal cell types, including smooth muscle cells (SMC) and fibroblasts. Endo-MT has been implicated in several pathological processes including cardiac fibrosis7 and pulmonary hypertension18, but its very existence is still controversial. It is thought to be driven by TGF in a SMAD-dependent and independent manner. Furthermore, factors leading to Endo-MT under pathologic conditions or preventing its occurrence in the normal vasculature have not been identified. We propose a novel paradigm stating that impaired endothelial FGF signaling leads to breakdown in vascular homeostasis resulting on one hand in Endo-MT (via decrease in let-7 mediated suppression of TGF signaling) and on the other in reduced vascular integrity due to the loss of VEGF signaling (decrease in FGF-dependent VEGFR2 expression) and increased permeability (increase VE-cadherin phosphorylation due to reduction in Shp2 expression). Thus impaired EC FGF signaling emerges as the common of denominator of key vascular pathologies. If correct, this hypothesis, and the sequence of events it is outlining, is not only critical to our understanding of the pathophysiology of number most common CV diseases but is equally important for the development of novel therapeutics.